A current challenge in the wind turbine industry is providing a blade design minimizing the fatigue and extreme loads and, consequently, allowing the reduction of the material, weight and prize of the blade.
The airfoil sections on the wind turbine blades in normal production operate at angles of attack ranging from low angles of attack, where the lift is low on the linear part of the lift versus angle of attack curve, to high angles of attack where the airfoil section is just before stall or in stall.
The variation of the aerodynamic loads can be measured as the oscillations of the aerodynamic lift and drag. The standard deviation statistic of a data set or time series is often used as a good indicator of the oscillation. A high standard deviation of lift and drag means that the variation of the aerodynamic loads is high resulting in higher fatigue loads.
Particularly in large blades the different sections of the blade are subject to different loads. In pitch controlled wind turbines, when the rated power is reached or at high wind speeds, the blades are pitched towards the feathered position reducing the lift coefficient to remove the excessive energy. In these situations it may happen that the root section of the blade has positive lift and the tip section has negative lift causing high fatigue loads, specially at the mid-span section of the blade.
In other operation situations, the turbine blades are moved towards feathered position in order to slow down the rotor an finally stop the wind turbine. In that case, negative lift is desired in the blades, but very large negative lift during the transition period could lead to large loads in the blades or any other component in the wind turbine.
Another situation where high fatigue loads are encountered is during the pitch variation introduced to minimise the effect of gusts and big scale atmospheric turbulence. When the turbulence involves an increase in the wind speed, the pitch of blade increases reducing the angle of attack so as to reduce the aerodynamic loads. The wind speed increase is usually followed by a wind speed decrease, which requires lower pitch angles and higher angles of attack to produce the same torque. During this process it may occur that part of the blade, usually the outboard, changes from positive angles of attack to negative ones, or vice-versa in a very short time. Since this means that part of the blade will be on the negative lift area of the lift vs. angle of attack curve, and will change to positive area in a very short period of time, from a load point of view, this process imposes on the blade a load that changes its direction with time.
This present invention is intended to provide a solution to these problems.